1. Field of the Invention
This invention relates to a heat exchanger, such as an evaporator for use in air-conditioning systems for automotive vehicles, and more particularly to a heat exchanger of this kind having outer surfaces thereof coated with hydrophilic coating layers.
2. Background Information
Conventionally, a heat exchanger of this kind has been proposed e.g. by Japanese Patent Publication (Kokoku) No. 60-45776, which comprises a plurality of tube elements, each of which is composed of a pair of stamped plates joined together to define flat refrigerant-evaporating passages therebetween, and a plurality of corrugated fins interposed between adjacent tube elements, the tube elements and the corrugated fins being superposed one upon another in an alternate manner, wherein the outer surfaces of the stamped plates and the corrugated fins are coated with hydrophilic coating layers. The hydrophilic coating layers generally contain approx. 80% of colloidal silica and approx. 20% of alkali silicate (water glass) K.sub.2 O.3SiO.sub.2. The hydrophilic coating layers improve the hydrophilic property of the surfaces of the corrugated fins and the tube elements, which concerns the resistance of condensate deposited thereon to the air flow.
In such a conventional heat exchanger, in general, the higher the colloidal silica content, the smaller the contact angle .theta. formed between the surface of a solid object and the surface of a liquid drop on the solid object (an angle formed at a point where the surface of a liquid drop is in contact with the surface of a solid object between a tangent to the liquid drop surface at the point and the solid object surface), as shown in FIG. 3a, i.e. the more desirable the hydrophilic property of the surfaces, whereas the smaller the colloidal silica content, the larger the contact angle .theta., as shown in FIG. 3b, i.e. the poorer the hydrophilic property of the same.
Although the colloidal silica contributes to enhancing the hydrophilic property of a material mixed therewith, it also has a strong adsorptivity. Therefore, smells, such as a smell of dust or an initial smell of a new manufactured article, are liable to be adsorbed by the colloidal silica, and the heat exchanger may omit the smells, e.g. when an air-conditioning system incorporating the heat exchanger and installed on an automotive vehicle is stopped, so that the smells, which may be offensive to occupants of the vehicle, are fed into the passenger compartment of the vehicle. In short, the colloidal silica can generate offensive smells. Therefore, there has been a demand for a hydrophilic coating layer which is excellent in hydrophilic property but is not liable to generate offensive smells.